This is unusual typesetting. You've put the line break in a sensible place, but I've never seen anybody write the operator on both lines. blah \cdot \\ blah or blah \\ \cdot blah are normal.
– David RicherbyJun 21 at 14:35

The brackets are surplus here, the fraction is a group by itself, so you don't need extra grouping. If I use brackets, I will enclose the whole formula to the right of the = sign into a pair of brackets. I also used \biggl[ and \biggr] for easy splitting. Finally, as @Sebastiano suggested, use \mathrm for text.

Note: you didn't tell us how thick the margins are, so I assumed 1in. If my answer is still too wide, uncomment the \small block.

If possible, I'd push the second line to the right so it began to the right of the product sign. That would visually cue that it's a part of the product (though the brackets and the presence of $j$s in the second line do make this semantically clear).
– David RicherbyJun 21 at 14:36

With multiline one can insert \\ at the locations where the line should be broken.

Multiletter subscripts ought not be set in math italics, the kerning is wrong. I suggest to warp them in \mathrm{}

The fontsize change needs to be outside the equation and without {} following it. \small was still a bit to large, but with \footnotesize the equation fits even without cheating and changing the margin sizes.